Relentless is a related term of fiendish.
As adjectives the difference between relentless and fiendish
is that relentless is unrelenting or unyielding in severity while fiendish is sinister; evil; conniving; in the manner of a fiend.
relentless
English
Adjective
(
en adjective)
Unrelenting or unyielding in severity
Unremitting, steady and persistent
Derived terms
* relentlessness
fiendish
English
Adjective
(
en adjective)
Sinister; evil; conniving; in the manner of a fiend.
Derived terms
* fiendishly
* fiendishness
Anagrams
* finished